Soybean Glycine max (L) is an important oil seed crop and a valuable field crop. However, it began as a wild plant. This plant and a number of other plants have been developed into valuable agricultural crops through years of breeding and development. The pace of the development of soybeans, into an animal foodstuff and as an oil seed has dramatically increased in the last one hundred years. Planned programs of soybean breeding have increased the growth, yield and environmental hardiness of the soybean germ plasm.
Due to the sexual reproduction traits of the soybean, the plant is basically self-pollinating. A self-pollinating plant permits pollen from one flower to be transferred to the same or another flower of the same plant. Cross-pollination occurs when the flower is pollinated with pollen from a different plant; however, soybean cross-pollination is a rare occurrence in nature.
Thus the growth and development of new soybean germplasm requires intervention by the breeder into the pollination of the soybean. The breeders' methods of intervening depends on the type of trait that is being bred. Soybeans are developed for a number of different types of traits including morphology (form and structure), phenotypic characteristics, and for traits like growth, day length, relative maturity, temperature requirements, initiation date of floral or reproductive development, fatty acid content, insect resistance, disease resistance, nematode resistance, fungal resistance, herbicide resistance, tolerance to various environmental factors like drought, heat, wet, cold, wind, adverse soil condition and also for yield. The genetic complexity of the trait often drives the selection of the breeding method.
Due to the number of genes within each chromosome, millions of genetic combinations exist in the breeders' experimental soybean material. This genetic diversity is so vast that a breeder cannot produce the same two cultivars twice using the exact same starting parental material. Thus, developing a single variety of useful commercial soybean germplasm is highly unpredictable, and requires intensive research and development.
The development of new soybeans comes through breeding techniques, such as: recurrent selection, mass selections, backcrossing, single seed descent and multiple seed procedure. Additionally, marker assisted breeding allows more accurate movement of desired alleles or even specific genes or sections of chromosomes to be moved within the germplasm that the breeder is developing. RFLP, RAPD, AFLP, SSR, SNP, SCAR, and isozymes are some of the forms of markers that can be employed in breeding soybeans or in moving traits into soybean germplasm. Other breeding methods are known and are described in various plant breeding or soybean textbooks.
When a soybean variety is being employed to develop a new soybean variety or an improved variety, the selection methods may include backcrossing, pedigree breeding, recurrent selection, marker assisted selection, modified selection and mass selection or a combination of these methods. The efficiency of the breeding procedure along with the goal of the breeding are the main factors for determining which selection techniques are employed. A breeder continuously evaluates the success of the breeding program and therefore the efficiency of any breeding procedures. The success is usually measured by yield increase, commercial appeal and environmental adaptability of the developed germ plasm.
The development of new soybean cultivars most often requires the development of hybrid crosses (some exceptions being initial development of mutants directly through the use of the mutating agent, certain materials introgressed by markers, or transformants made directly through transformation methods) and the selection of progeny. Hybrids can be achieved by manual manipulation of the sexual organs of the soybean or by the use of male sterility systems. Breeders often try to identify true hybrids by a readily identifiable trait or the visual differences between inbred and hybrid material. These heterozygous hybrids are then selected and repeatedly selfed and reselected to form new homozygous soybean lines.
Mass and recurrent selection can be used to improve populations. Several parents are intercrossed and plants are selected based on selected characteristics like superior yield or excellent progeny resistance. Outcrossing to a number of different parents creates fairly heterozygous breeding populations.
Pedigree breeding is commonly used with two parents that possess favorable, complementary traits. The parents are crossed to form a F1 hybrid. The progeny of the F1 hybrid is selected and the best individual F2s are selected; this selection process is repeated in the F3 and F4 generations. The inbreeding is carried forward and at approximately F5-F7 the best lines are selected and tested in the development stage for potential usefulness in a selected geographic area.
In backcross breeding a genetic allele or loci is often transferred into a desirable homozygous recurrent parent. The trait from the donor parent is tracked into the recurrent parent. The resultant plant is bred to be like the recurrent parent with the new desired allele or loci.
The single-seed descent method involves use of a segregating plant population for harvest of one seed per plant. Each seed sample is planted and the next generation is formed. When the F2 lines are advanced to approximately F6 or so, each plant will be derived from a different F2. The population will decline due to failure of some seeds, so not all F2 plants will be represented in the progeny.
New varieties must be tested thoroughly to compare their development with commercially available soybeans. This testing usually requires at least two years and up to six years of comparisons with other commercial soybeans. Varieties that lack the entire desirable package of traits can be used as parents in new populations for further selection or are simply discarded. The breeding and associated testing process is 8 to 12 years' of work prior to development of a new variety. Thousands of varietal lines are produced but only a few lines are selected in each step of the process. Thus the breeding system is like a funnel with numerous lines and selections in the first few years and fewer and fewer lines in the middle years until one line is selected for the final development testing.
The selected line or variety will be evaluated for its growth, development and yield. These traits of a soybean are a result of the variety's genetic potential interacting with its environment. All varieties have a maximum yield potential that is predetermined by its genetics. This hypothetical potential for yield is only obtained when the environmental conditions are near perfect. Since perfect growth conditions do not exist, field experimentation is necessary to provide the environmental influence and to measure its effect on the development and yield of the soybean. The breeder attempts to select for an elevated soybean yield potential under a number of different environmental conditions.
Selecting for good soybean yield potential in different environmental conditions is a process that requires planning based on the analysis of data in a number of seasons. Identification of the varieties carrying a superior combination of traits, which will give consistent yield potential, is a complex science. The desirable genotypic traits in the variety can often be masked by other plant traits, unusual weather patterns, diseases, and insect damage. One widely employed method of identifying a superior plant with such genotypic traits is to observe its performance relative to commercial and experimental plants in replicated studies. These types of studies give more certainty to the genetic potential and usefulness of the plant across a number of environments.
In summary, the goal of the soybean plant breeder is to produce new and unique soybeans and progeny of the soybeans for farmers' commercial crop production. To accomplish this, the plant breeder painstakingly crosses two or more varieties or germ plasm. Then the results of this cross are repeatedly selfed or backcrossed to produce new genetic patterns. Newer avenues for producing new and unique genetic alleles in soybeans include introducing (or creating) mutations or transgenes into the genetic material of the soybean are now in practice in the breeding industry. These genetic alleles can alter pest resistance such as disease resistance, insect resistance, nematode resistance, herbicide resistance, or they can alter the plant's environmental tolerances, or its seeds fatty acid compositions, the amount of oil produced, and/or the amino acid/protein compositions of the soybean plant or its seed.
The traits a breeder selects for when developing new soybeans are driven by the ultimate goal of the end user of the product. Thus if the goal of the end user is to resist a certain plant disease so overall more yield is achieved, then the breeder drives the introduction of genetic alleles and their selection based on disease resistant levels shown by the plant. On the other hand, if the goal is to produce specific fatty acid composition, with for example a high level of oleic acid and/or a lower level of linolenic acid, then the breeder may drive the selection of genetic alleles/genes based on inclusion of mutations or transgenes that alter the levels of fatty acids in the seed. Reaching this goal may allow for the acceptance of some lesser yield potential or other less desirable agronomic trait.
The new genetic alleles being introduced into soybeans are widening the potential uses and markets for the various products and by-products of the oil from seed plants such as soybean. A major product extracted from soybeans is the oil in the seed. Soybean oil is employed in a number of retail products such as cooking oil, baked goods, margarines and the like. Another useful product is soybean meal, which is a component of many foods and animal feedstuffs.